The present invention relates to a flow meter, especially for measuring fluid flow passing through large diameter conduits, in which, in a conduit section through which the fluid stream passes, a rod-shaped vortex generating body extending transverse to the direction of fluid flow is arranged, from which Karman vortices detach themselves, the frequency of which corresponds to the speed of the fluid passing through the conduit, which speed is measured by a feeler element.
In such a flow meter, which is constructed as a vortex counter, there exists between the frequency of the produced Karman vortices and the fluid flow speed the relationship EQU f=S.multidot.v/b
in which f is the vortex detaching frequency, v the speed of the fluid flow, b the characteristic width of the vortex generating body, and S the Strouhal number. The Strouhal number S is determined by the equation EQU S=(f.multidot.b/v),
wherein v is the speed of the fluid, f is the vortex frequency, and b is the width of the vortex generating body. The cross section of the vortex generating body determines the aforementioned Strouhal number, which over a wide range of fluid speed is constant, so that also the relationship between the frequency of the created Karman vortices and of the fluid stream speed is correspondingly linear. Optimum vortex signals with a high signal quality and a large linear region can be obtained by corresponding selection of the cross section of the vortex generating body in relationship to the cross section of the conduit through which the fluid passes.
As will be clear from the first of the equations above, the vortex detaching frequency drops with increasing width of the vortex generating body. If one uses, to obtain a geometric similarity during mounting of the vortex countar into conduit with increasingly larger diameter, with the same or similar cross section of the vortex generating body and the same ratio of the cross section of the body to the cross section of the conduit, then the Strouhal number will remain the same, but the frequency of the Karman vortices will drop with increasing width of the body, so that the determination of the vortex frequency will become more difficult with the increase of the diameter of the conduit. Thus, for instance, at a conduit diameter of 250 mm a satisfactory vortex signal can still be obtained, whereas at a conduit diameter of 600 mm the vortex frequency is so low that the evaluation of the same in the connected electronic computing device meets with great difficulty.
If the width b of the vortex generating body to be built into large-diameter conduits would be reduced, then according to the above equation the vortex frequency f could be increased; however, the stability of the vortex path would be endangered, since the ratio of slenderness of the vortex generating body which extends through the whole cross section of the conduit would be too great. In this case, the vortices would not detach themselves in the form of cylinders from the total length of the vortex generating body, but due to the small coupling of the fluid different detaching regions would occur in which the detachment of the vortices would not be in phase, whereby disturbing transverse streams would be formed, which would impair the quality of the signals.